1. Field of the Invention
The present invention relates to an aqueous vesicle dispersion utilizing a readily available nonionic surfactant and having a surface charge. More specifically, it relates to an aqueous vesicle dispersion having an improved dispersion stability and adsorbability of the vesicle particles and which is capable of including a hydrophilic or hydrophobic effective or active component in a state isolated from an aqueous dispersion medium.
2. Description of the Related Art
It is known in the art that an amphiphatic substance can form vesicles in water. For example, vesicles such as liposomes based on phospholipids and ufasomes based on unsaturated fatty acids are present in natural substances. Attempts have been made to use these natural vesicles in the fields of cosmetics and pharmaceuticals, since these types of vesicles are stable dispersions and have no safety problems. However, these natural vesicles are not suitable for use in large volume consumption because of their relatively high cost.
Recently, vesicles or niosomes utilizing readily available nonionic surfactants have been found. For example, it has been reported in Japanese Unexamined Patent Publication (Kokai) No. 52-6375 that vesicle dispersions are formed from nonionic surfactants having the general formula: ##STR1## wherein R represents an aliphatic hydrocarbon group having 12 to 30 carbon atoms and n represents an integer of from 1 to 6. It has also been reported in J. Colloid Interface Sci., Vol. 82, No. 2, 401-417 (1981) that vesicles are formed from ethylene oxide addition products of glycerin dialkyl ethers or ethylene oxide addition products of myristic acid stearyl amides. On the other hand, it has been reported in Nippon Kagaku Kaishi, 1981 (11), p 1691-1696 that polyoxyethylene hydrogenated castor oil ethers and polyoxyethylene sorbitol tetraoleate form concentric lamella type liquid crystals.
Vesicles can be considered to be special forms of concentric lamella type liquid crystals. However, vesicles are different from the concentric lamella type liquid crystals in that water or an aqueous solution is contained in a substantial amount in hydrophilic cavities formed in the inside of bimolecular or multiple layer membranes (or films) formed from a surfactant. These cavities are larger than the space between the bimolecular membranes found in regular lamella type liquid crystals.
It is necessary that the surfactant molecules be oriented so as to form lamella bimolecular membranes having a curvature capable of readily forming vesicles. However, although the above-mentioned polyoxyethylene hydrogenated castor oil ethers form concentric lamella type liquid crystals, the polyoxyethylene hydrogenated castor oil ethers cannot form uniform state concentric lamella type liquid crystals and clear vesicle structures since the polyoxyethylene hydrogenated castor oil ethers are mixtures of plural compounds having different ethylene oxide addition mole numbers.
The present inventors have developed a nonionic surfactant type vesicle dispersion which has been found to be acceptable, by official authorities concerned, for use in the fields of foods, drugs, and cosmetics and which can be produced from a readily available surfactant (U.S. Pat. No. 4,536,324 noted above). This nonionic surfactant type vesicle dispersion is formed by adding sorbitan polyesters of long-chain fatty acids to nonionic surfactants, i.e., polyoxyethylene castor oil ethers and/or polyoxyethylene hydrogenated castor oil ethers in a certain ratio to cause the orientation of the nonionic surfactants with a curvature such that vesicles are readily formed. However, the nonionic surfactant type vesicle dispersions thus obtained are still not satisfactory from the viewpoints of dispersion stability and adsorbability of the vesicle particles.